1. Field of the Invention
The present invention relates to a projection zoom lens which is provided in, for example, a projection display device and includes six lens groups, four of which are moved, and a projection display device including the projection zoom lens, and more particularly, to a projection zoom lens and a projection display device that are suitable to project an image onto a large screen in, for example, a movie theater.
2. Description of the Related Art
In recent years, projectors (projection display devices) that use light valves, such as liquid crystal display devices or DMD display devices, and have a relatively large back focal length have come into widespread use (see JP-A-8-201690).
In recent years, projectors that can be applied to a large screen and project a high-definition image have been used in, for example, the movie theater.
The cinema projector is a three-panel type including three reflective liquid crystal display devices or three DMDs and requires a large back focal length and high telecentricity.
In general, a value obtained by dividing the projection distance by the width of the screen is called a throw ratio. The screen size and the distance (projection distance) from the screen to a projection room depend on the movie theaters. Therefore, a lens corresponding to the throw ratio of each movie theater is needed to project an image with a size suitable for each movie theater. It is inefficient to prepare the individual lenses corresponding to various throw ratios in terms of costs. In order to solve the above-mentioned problems, a zoom lens can be used to respond to a wide throw ratio range.
However, in many lenses for a projector according to the related art, the numerical aperture thereof is changed during zoom (hereinafter, in some cases, ‘Fno.’ is used instead of the numerical aperture). In general, the Fno. at the telephoto end is larger than that at the wide angle end. Therefore, in the zoom lens, even when the screen sizes of the movie theaters are equal to each other, the image projected onto the screen of the movie theater with a high throw ratio is darker than that projected onto the screen of the movie theater with a low throw ratio.
As the zoom ratio of the lens increases, the compatibility of the lens increases. However, in the lens for a projector according to the related art, as the zoom ratio increases, a variation in Fno. tends to increase. Therefore, the lens is not suitable for the movie theater.
JP-A-2002-122782 discloses a zoom lens capable of solving the problems that the Fno. is changed with the variation in the zoom ratio and the lens for a projector is not suitable for the movie theater. However, since the lens disclosed in JP-A-2002-122782 is a varifocal lens, telecentricity is not maintained to be constant when power varies, or after the lens is provided, the adjustment of an operation is complicated when power needs to vary.
JP-A-2007-241184 discloses a zoom lens in which the zoom ratio increases while Fno. is maintained to be constant during zoom. The zoom lens disclosed in JP-A-2007-241184 has a small back focal length that is not suitable for a cinema zoom lens and uses a cemented lens. Therefore, when the zoom lens is provided in a projection projector apparatus using a light source, such as a xenon lamp that emits light with very high intensity, an adhesive for bonding lenses is significantly altered and deteriorated by the high-intensity light, which causes a reduction in the performance of the lens.
In order to solve the above-mentioned problems, the inventors have proposed a telecentric projection zoom lens which has an appropriate back focal length, a constant Fno. during zoom, a high zoom ratio, and high compatibility, and does not use a cemented lens (see Japanese Patent Application Nos. 2007-304317 and 2007-304318).
The projection zoom lens disclosed in Japanese Patent Application. No. 2007-304317 (hereinafter, referred to as ‘Proposed invention 4’) includes five lens groups. Among the five lens groups, three lens groups are moved, and an aperture diaphragm that is fixed when power varies is provided between the fourth lens group and the fifth lens group.
The projection zoom lens disclosed in Japanese Patent Application No. 2007-304318 (hereinafter, referred to as ‘Proposed invention 5’) includes six lens groups. Among the six lens groups, four lens groups are moved, and an aperture diaphragm that is independently moved when power varies is provided between the fourth lens group and the fifth lens group.
The zoom lens according to Proposed invention 5 has an advantage in that it can obtain an angle of view wider than that according to Proposed invention 4. However, the moving group is arranged close to the reduction side. Therefore, when the outside diameters of the lenses including mechanisms are compared with each other, the outside diameter of the lens arranged on the reduction side in the zoom lens according to Proposed invention 5 is more than that in the zoom lens according to Proposed invention 4.
In general, a coupling mechanism for coupling the projection display device and the lens is provided in an outer circumferential portion of the lens. However, when the lens is coupled to the existing cinema projection display device, the coupling mechanism is generally provided closer to the reduction side than the position of the aperture diaphragm (in the zoom lens according to Proposed invention 5, the position of the aperture diaphragm position at the wide angle end).
In the existing projection display device, the size of a space for arranging the projection lens has already been determined. Therefore, in order to provide a newly proposed projection lens in the existing device, at least the lens arranged closer to the reduction side than the position of the coupling mechanism needs to have a sufficient outside diameter to be accommodated in the space for arranging the lens in the device.
However, the outside diameter of the lens arranged closer to the reduction side than the position of the coupling mechanism in the zoom lens according to Proposed invention 5 is more than that in the zoom lens according to Proposed invention 4. Therefore, it is difficult to attach the zoom lens according to Proposed invention 4 to the existing projection display device that can be attached. In the zoom lens disclosed in JP-A-2007-241184, since the moving group is arranged closer to the reduction side than the position of the aperture diaphragm, the outside diameter of the reduction-side lens increases.